Steel pipes for marine structures and line pipes are subjected to girth welding in order to join the steel pipes in a process for forming the structures. Girth welding is welding in the circumferential direction of the steel pipes and is a process which is indispensable in the process for forming the structures. Therefore, from the viewpoint of safety of the structures, such steel pipes are required to be excellent in terms of the toughness of a weld zone formed as a result of performing girth welding, in addition to being excellent in terms of the toughness of a base material.
Girth welding mentioned above usually involves low- to medium-heat input multi-pass welding (also referred to as multilayer welding). In the case of multi-pass welding, a heat affected zone is composed of regions which have been subjected to various thermal histories.
In a bond, which has been formed in the first welding heat cycle of multilayer welding, that is, a coarse-grain region in the vicinity of the boundary between a weld metal and a heat affected zone, island martensite (also referred as MA (an abbreviation for Martensite Austenite constituent)) is formed in a region which is reheated at a temperature in a temperature range for forming a ferrite-austenite dual phase (hereinafter, also simply referred to as a dual-phase temperature range) due to a subsequent welding heat cycle (also referred to as coarse-grain region reheated in a dual-phase temperature range). In the case where island martensite is formed, there is a significant decrease in toughness. The coarse-grain region reheated in a dual-phase temperature range is the region having the lowest toughness in the heat affected zone formed as a result of performing multilayer welding.
As a countermeasure against a decrease in toughness in a coarse-grain region reheated in a dual-phase temperature range, a technique in which the strength of base material is increased by adding Cu, while inhibiting the formation of MA as a result of decreasing C content and decreasing Si content, has been proposed (for example, Patent Literature 1).
In addition, since a bond is exposed to a high temperature immediately below the melting point, an austenite grain size increases the most in the bond. In addition, since the microstructure in the bond tends to transform into an upper bainite structure due to subsequent cooling, there is a decrease in toughness.
As a measure for increasing the toughness of a bond, a technique in which TiN is finely dispersed in steel in order to inhibit the growth of austenite grain or in order to utilize the TiN as a ferrite nucleation site has been put into practice.
Patent Literature 2 discloses a technique in which the toughness of a heat affected zone is increased by the fine dispersion of ferrite nucleation sites as a result of crystallizing CaS. In addition, Patent Literature 2 proposes a technique in which the technique described in Patent Literature 2 is combined with a technique in which Ti oxides are dispersed (for example, Patent Literature 3) or combined with the ferrite nucleation capability of BN and the dispersion of oxides. Moreover, Patent Literature 2 also proposes a technique in which high toughness is achieved by adding Ca and REM in order to control the shape of sulfides.
As a criterion for evaluating the toughness of steel, absorbed energy in a Charpy test has mainly been used to date. There is a case where it is required to conduct a CTOD test (an abbreviation for Crack Tip Opening Displacement test) in order to evaluate the toughness of steel for increased reliability. In a CTOD test, resistance to the occurrence of a brittle fracture is evaluated by performing a three-point bending test on a test piece having a fatigue crack in an evaluated portion and by determining the amount of opening (the amount of plastic deformation) at a crack bottom immediately before the occurrence of a fracture.
CTOD performance indicates the toughness of a small region at the crack bottom. In order to satisfy the strict requirement for the CTOD performance of a bond formed as a result of performing girth welding, it is necessary to increase the toughness of a coarse-grain region reheated in a dual-phase temperature range which is a region in a heat affected zone and in which there is a decrease in toughness.